Review on soil water movement affected by soil water repellency

Abstract: Soil water repellency (SWR) is a normal property with major repercussions for plant growth, surface and subsurface hydrology, and for soil erosion. Important advances have been made since the late 1960s in identifying the range of environments affected by SWR, its characteristics and its hydro-geomorphological impacts. In this review, we outlined the concept of SWR, summarized the commonly used methods of soil water content (SWC) measurement and classification criterion, but focused particularly on recent advances in identifying the impacts of SWR on water movements, and indicated the existing research gaps. Significant advances relating to the impacts of SWR on the hydraulic properties as well as the dynamics of water infiltration and evaporation in these unique systems were discussed. SWR affected the matric potential through the contact angle, and shifts the water entry values. The wetting branch of water retention curves is strongly affected by the contact angle, in contrast, effects for the drying branch are minimal, SWR can resist or retard surface water infiltration. Besides the retardation or resistance of surface water infiltration, water repellent soils have been associated with fingered flow. Fingers are known to have a distinctive nonmonotonic saturation profile, with water accumulation behind the wetting front (tip) and lower saturation above it (tail). This nonmonotonic saturation profile is known as saturation overshoot. Saturation overshoot is associated with capillary pressure overshoot and has been confirmed to be the cause for gravity driven fingering. However, the saturation overshoot cannot be described by the Richards equation, the primary unsaturated flow equation. Richards’ equation in its basic form, along with the standard (monotonic) pressure-saturation relations is incompatible with the saturation overshoot for fingered flow. Modifications of Richards’ equation and other approaches have been proposed to account for and describe the nonmonotonicity of the saturation field. In comparison with numerous work on infiltration, studies on evaporation are relatively less but show uniform results, namely SWR suppresses soil evaporation. Percolation-based morphological pore network modeling of evaporation confirms the results. Experimental and modeling results suggest that water repellency affects liquid phase continuity in partially wettable porous media and reduces the evaporation characteristic length relative to capillarity effects. Major research gaps, however, remain in (a) identifying the exact role of, and the interactions between the variables controlling the development and effectiveness of flow pathways through repellent soils; (b) establishing and improving the models with new mechanisms with consideration of the effects of SWR in controlling fingers, and proposing the relevant analytical and numerical methods for solving these new models. Improved understanding of effects of SWR will enable its overall roles in surface and subsurface hydrological and erosion processes to become more clearly defined.